JP7085147B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery.

A non-aqueous electrolyte secondary battery is preferably used as a portable power source, a high output power source for mounting on a vehicle, or the like because it is lightweight and has a high energy density. In this non-aqueous electrolyte secondary battery, a storage element having a positive electrode and a negative electrode insulated by a separator or the like is provided with a spiral electrode body in which a columnar or elliptical columnar layer is wound in one battery case. There is. Here, the positive electrode and the negative electrode are designed so that the widthwise dimension of the negative electrode is wider than the widthwise dimension of the positive electrode in order to prevent the precipitation of lithium ions on the negative electrode. For example, Patent Document 1 discloses that in such a secondary battery, an insulating layer containing an inorganic filler is provided on the surface of the positive electrode current collector along the end portion of the positive electrode active material layer. It is described that this insulating layer can prevent a short circuit between the positive electrode current collector and the end portion of the negative electrode active material layer facing the positive electrode current collector.

JP-A-2017-143004

By the way, among the positive electrode current collectors, the current collector portion where the current collector is performed is a non-coated portion in which the active material layer is not formed. Since the non-coated portion of the positive electrode is close to the current collecting terminal and has a high current density, there is a problem that the high potential of the positive electrode tends to cause an oxidation state and generate heat. In particular, a secondary battery having an insulating layer on a positive electrode current collector tends to have a high temperature because the current collector is covered with the insulating layer. According to the studies by the present inventors, it has been found that even a secondary battery having an insulating layer in the non-coated portion of the positive electrode may cause a short circuit due to heat generation of the battery, for example.
The present invention has been made in view of such circumstances, and an object thereof is to more preferably suppress a short circuit between a positive electrode current collector and a negative electrode active material layer even when the battery generates heat. It is to provide a non-aqueous electrolyte secondary battery that can be used.

As a solution to the above problems, the technique disclosed herein provides a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator that insulates the positive electrode and the negative electrode, and a non-aqueous electrolyte. .. In this non-aqueous electrolyte secondary battery, the positive electrode is a positive electrode current collector, a positive electrode active material layer provided on a part of the surface of the positive electrode current collector and containing a positive electrode active material, and a surface of the positive electrode current collector. It is provided with an insulating layer which is another part and is provided adjacent to the positive electrode active material layer. The insulating layer contains an inorganic filler and a binder, and the insulating layer evaluation sample formed in a square shape having a side of 5 cm is heat-treated at 150 ° C. for 1 hour. The heat shrinkage in one direction parallel to the surface of the evaluation sample is 13% or less.

Since a general separator is stretched, it softens or melts easily (for example, 15% or more, typically close to 30%) by being exposed to the above heating conditions without any restrictions. do. Further, since a general binder is also softened or melted at the above temperature, the insulating layer of the positive electrode is easily exposed to the above heating conditions without being restricted (for example, 15% or more, typically 20). %) Can shrink heat. As a result, when the widthwise end (which may be a corner) of the negative electrode active material layer comes into contact with the insulating layer of the positive electrode, the insulating layer is easily deformed and the positive electrode current collector and the negative electrode active material layer are short-circuited. Can be. On the other hand, according to the configuration disclosed here, the heat shrinkage rate in the plane direction of the insulating layer is suppressed to 13% or less even when heated to 150 ° C. The heat shrinkage of the insulating layer that achieves such a heat shrinkage rate is suppressed by the presence of the inorganic filler. Such an inorganic filler has a configuration in which it is difficult for the negative electrode active material layer to deform the insulating layer and reach the positive electrode current collector even when the separator is thermally shrunk and comes into contact with the negative electrode active material layer. It has been realized. As a result, even when the secondary battery generates heat, it is possible to suitably suppress a short circuit between the positive electrode current collector and the negative electrode active material layer.

In a preferred embodiment of the non-aqueous electrolyte secondary battery according to the present technique, the inorganic filler contains plate-like particles. The inorganic filler preferably contains plate-like particles having an average aspect ratio of 3 or more. According to such a configuration, even when the insulating layer is heated to 150 ° C. as described above, the heat shrinkage rate can be suitably suppressed to 13% or less. Further, for example, even when an internal short circuit occurs, it is preferable because the effect of reducing the area of the internal short circuit and suppressing the expansion can be obtained.

In a preferred embodiment of the non-aqueous electrolyte secondary battery according to the present technique, the inorganic filler is at least one of boehmite powder and alumina powder. The inorganic filler is preferably boehmite powder. By using such a powder as the inorganic filler, an insulating layer having excellent heat resistance can be easily produced. In particular, it is preferable to use boehmite powder, which has a lower Mohs hardness than alumina, because it is possible to suppress the generation of metallic foreign matter by scraping the equipment contact portion in the manufacturing process.

In a preferred embodiment of the non-aqueous electrolyte secondary battery according to the present technique, the ratio of the binder to the total of the inorganic filler and the binder in the inorganic filler is less than 30% by mass. According to such a configuration, even when the insulating layer is heated to 150 ° C. as described above, the heat shrinkage rate can be easily reduced to 13% or less.

In a preferred embodiment of the non-aqueous electrolyte secondary battery according to the present technique, the average thickness of the insulating layer is 10 μm or less. In the secondary battery, from the viewpoint of weight reduction and cost reduction, it is preferable that the insulating layer provided on the positive electrode current collector is small in bulk and quantity. As long as the insulating layer achieves the heat shrinkage rate of 13% or less, even when the average thickness is 10 μm or less, for example, a short circuit caused by a metal foreign substance and its expansion can be suitably suppressed. .. As a result, a secondary battery that is safe, lightweight, and low in cost is provided.

The above non-aqueous electrolyte secondary battery is provided as having high safety by suppressing a short circuit between the positive electrode current collector (non-coated portion) and the negative electrode active material layer even at a high temperature of, for example, 150 ° C. .. For safety at such a high temperature, for example, the battery itself has a laminated structure (including a laminated electrode body and a wound electrode body) in which a plurality of power storage elements are laminated, and repeatedly charges and discharges a large current at a high rate. It can be suitably applied to a secondary battery for applications where the temperature tends to be high due to charging and discharging. In addition, the above-mentioned high safety can be suitably applied to a secondary battery for applications that are closely used by humans and require high safety. Therefore, the non-aqueous electrolyte secondary battery disclosed herein can be particularly preferably used as a driving power source (main power source) for a vehicle, particularly as a driving power source for a hybrid vehicle, a plug-in hybrid vehicle, or the like.

It is a notch perspective view schematically showing the structure of the non-aqueous electrolyte secondary battery which concerns on one Embodiment. It is a partially developed view explaining the structure of the winding type electrode body. It is sectional drawing of the main part of the non-aqueous electrolyte secondary battery which concerns on one Embodiment. (A) and (b) are schematic cross-sectional views illustrating the influence of the shape of the inorganic filler contained in the insulating layer. It is sectional drawing which explains the structure of the electrode body produced in an Example. It is a perspective view explaining the structure of the metal foreign matter piece used in an Example.

Hereinafter, an embodiment of the non-aqueous electrolyte secondary battery disclosed herein will be described. It should be noted that matters other than those specifically mentioned in the present specification (for example, the configuration of the insulating layer, etc.) and necessary for carrying out the present invention (for example, the structure of the secondary battery which does not characterize the present invention) and the like. The manufacturing process, etc.) can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art. Further, the dimensional relations (length, width, thickness, etc.) in the drawings shown below do not necessarily reflect the actual dimensional relations. In the present specification, the notation "A to B" indicating a numerical range means "A or more and B or less".

As used herein, the term "non-aqueous electrolyte secondary battery" refers to a general battery that uses a non-aqueous electrolyte as a charge carrier and can be repeatedly charged and discharged as the charge carrier moves between the positive and negative electrodes. The electrolyte in the non-aqueous electrolyte secondary battery may be, for example, a non-aqueous electrolyte solution, a gel-like electrolyte, or a solid electrolyte. Such a non-aqueous electrolyte secondary battery includes a battery generally called a lithium ion battery, a lithium secondary battery, or the like, a lithium polymer battery, a lithium ion capacitor, or the like. Hereinafter, the techniques disclosed herein will be described by taking as an example the case where the non-aqueous electrolyte secondary battery is a lithium ion secondary battery.

[Lithium-ion secondary battery]
FIG. 1 is a notched perspective view showing the configuration of a lithium ion secondary battery (hereinafter, simply referred to as “secondary battery” or the like) 1 according to an embodiment. The lithium ion secondary battery 1 is configured such that a wound electrode body 20 including a positive electrode 30, a negative electrode 40, and a separator 50 is housed in a battery case 10 together with a non-aqueous electrolytic solution (not shown). .. W in the figure indicates the width direction of the battery case 10 and the winding type electrode body 20, and is a direction corresponding to the winding axis WL of the winding type electrode body 20 shown in FIG. As shown in FIG. 2, the electrode body 20 is configured by laminating a separator 50, a negative electrode 40, a separator 50, and a positive electrode 30 in this order. FIG. 3 is a cross-sectional view of a main part of the electrode body 20.

The positive electrode 30 includes a positive electrode current collector 32, a positive electrode active material layer 34, and an insulating layer 36.
The positive electrode active material layer 34 is a porous body containing the positive electrode active material and may be impregnated with the electrolytic solution. The positive electrode active material releases lithium ions, which are charge carriers, into or occludes the electrolytic solution. The positive electrode active material layer 34 can additionally contain a conductive material and trilithium phosphate (Li ₃ PO ₄ ; hereinafter, simply referred to as “LPO”). The positive electrode active material layer 34 is provided on a part of the surface (one side or both sides) of the positive electrode current collector 32. The positive electrode current collector 32 is a member that holds the positive electrode active material layer 34 and supplies and recovers electric charges to the positive electrode active material layer 34. The positive electrode current collector 32 is preferably electrochemically stable in the positive electrode environment in the battery, and is more preferably made of a conductive member made of a metal having good conductivity (for example, aluminum, aluminum alloy, nickel, titanium, stainless steel, etc.). It is composed.

The positive electrode active material layer 34 is typically bonded to the positive electrode current collector 32 while the granular positive electrode active material is bonded to each other together with the conductive material by a binder (binding agent). As the positive electrode active material, various materials conventionally used as the positive electrode active material of the lithium ion secondary battery can be used without particular limitation. Suitable examples include lithium nickel oxide (eg LiNiO ₂ ), lithium cobalt oxide (eg LiCoO ₂ ), lithium manganese oxide (eg LiMn ₂ O ₄ ) and composites thereof (eg LiNi _0.5 Mn ₁ ). _.5 O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) and other oxide particles containing lithium and transition metal elements as constituent metal elements (lithium transition metal oxide particles) and phosphoric acid Examples thereof include phosphate particles containing lithium and a transition metal element as constituent metal elements such as lithium manganese (LiMnPO ₄ ) and lithium iron phosphate (LiFePO ₄ ). Such a positive electrode active material layer 34 may be, for example, a positive electrode active material, a conductive material, a binder (for example, an acrylic resin such as a methacrylic acid ester polymer, a vinylidene halide resin such as polyvinylidene fluoride (PVdF), or a polyethylene oxide (for example). A positive electrode paste obtained by dispersing polyalkylene oxide such as PEO) in an appropriate dispersion medium (for example, N-methyl-2-pyrrolidone) is supplied to the surface of the positive electrode current collector 32, and then dried and dispersed. It can be produced by removing the medium. In the configuration including the conductive material, for example, a carbon material such as carbon black (typically acetylene black or Ketjen black), activated carbon, graphite, carbon fiber or the like can be preferably used as the conductive material. Any one of these may be used alone or in combination of two or more.

The average particle size (D ₅₀ ) of the positive electrode active material particles is not particularly limited, and is typically 1 μm or more, preferably 3 μm or more, for example 5 μm or more, typically 15 μm or less, preferably 10 μm or less, for example 8 μm. It is as follows. The ratio of the positive electrode active material to the entire positive electrode active material layer 34 may be about 75% by mass or more, typically 80% by mass or more, for example 85% by mass or more, and typically 99% by mass or less, for example. It can be 95% by weight or less. The ratio of the conductive material in the positive electrode active material layer 34 is typically 1 part by mass or more, preferably 3 parts by mass or more, for example, 5 parts by mass or more, and typically 5 parts by mass or more, with respect to 100 parts by mass of the positive electrode active material. It is 15 parts by mass or less, preferably 12 parts by mass or less, for example, 10 parts by mass or less. The ratio of the binder in the positive electrode active material layer 34 is typically 0.5 parts by mass or more, preferably 1 part by mass or more, for example 1.5 parts by mass or more, with respect to 100 parts by mass of the positive electrode active material. It can be 10 parts by mass or less, preferably 8 parts by mass or less, for example, 5 parts by mass or less. The thickness of the positive electrode active material layer 34 after pressing (average thickness; the same applies hereinafter) is typically 10 μm or more, for example 15 μm or more, and typically 50 μm or less, 30 μm or less, for example 25 μm. It can be as follows. The density of the positive electrode active material layer 34 is not particularly limited, but is typically 1.5 g / cm ³ or more, for example 2 g / cm ³ or more, and 3 g / cm ³ or less, for example 2.5 g / cm ³ . It can be as follows.
In the present specification, the "average particle size" is a cumulative 50% particle size (D ₅₀ ) in the volume-based particle size distribution obtained by the laser diffraction / scattering method, unless otherwise specified. Further, the particle size corresponding to the cumulative 10% from the small particle size side in the particle size distribution is called D ₁₀ , the particle size corresponding to the cumulative 90% is called D ₉₀ , and the maximum frequency diameter is called D _max .

According to the studies by the present inventors so far, in the configuration where the positive electrode active material layer 34 contains LPO, the LPO reacts with the acid generated by the oxidative decomposition of the electrolytic solution and the like, and the phosphate ion (PO). It becomes ₄ ^3- ) and elutes. The phosphate ion reaches the negative electrode and can form a film that suitably suppresses the exothermic reaction of the negative electrode to suitably improve the overcharge resistance of the secondary battery. From the viewpoint of suppressing heat generation of the secondary battery 1, a configuration in which the positive electrode active material layer 34 contains LPO may be a preferred embodiment. When the positive electrode active material layer 34 contains LPO, the ratio of LPO is the positive electrode active material 100 from the viewpoint of achieving both the effect of improving the overcharge resistance by LPO and the increase in viscosity and productivity of the positive electrode paste at the time of producing the positive electrode. It is preferable that the LPO is 0.88 to 8.8 parts by mass with respect to the mass part. Further, the specific surface area of the LPO is preferably 0.9 to 20.3 m ² / g from the viewpoint of achieving both improvement of overcharge resistance and reduction of reaction resistance. The average particle size of the LPO is preferably 1 μm or more, more preferably 2 μm or more, for example, 2.5 μm or more, preferably 30 μm or less, more preferably 8 μm or less, and for example, 5 μm or less. The D ₉₀ of LPO is preferably 60 μm or less, more preferably 40 μm or less, and may be 20 μm or less. The D ₁₀ of LPO is preferably 0.3 μm or more, more preferably 0.6 μm or more, and may be 0.8 μm or more. The D _max is preferably 80 μm or less, more preferably 60 μm or less, and preferably 50 μm or less.

The insulating layer 36 contains an inorganic filler and a binder, and has electrical insulating properties. Such an insulating layer 36 is typically formed by binding inorganic fillers to each other and to the positive electrode current collector 32 by a binder. The insulating layer 36 may be a porous layer that allows the passage of charge carriers. As shown in FIGS. 2 and 3, the insulating layer 36 is a part of the surface (one side or both sides) of the positive electrode current collector 32 and is provided in a region adjacent to the positive electrode active material layer 34. In other words, the insulating layer 36 is provided along the widthwise end of the positive electrode active material layer 34. The insulating layer 36 is provided in a region adjacent to the positive electrode active material layer 34 (a region in which the positive electrode active material layer 34 is not formed) and at least in a region facing the negative electrode active material layer 44. In one example, as shown in FIG. 3, the insulating layer 36 may project outward (on the left side in the figure) by the dimension α from the negative electrode active material layer 44 in the width direction. The dimension α is such that even if the negative electrode active material layer 44 is displaced, the negative electrode active material layer 44 and the positive electrode current collector 32 face each other only through the separator 50. The dimensions are designed so that the insulating layer 36 can sufficiently cover the end portion of the material layer 44. Further, the dimension α may be designed to have a dimension that can sufficiently cover the end portion of the negative electrode active material layer 44 even when the insulating layer 36 is thermally shrunk in a high temperature environment. The dimension α is designed so that the insulating layer 36 does not protrude from the end of the separator 50 in the width direction in order to avoid poor foil collection of the current collector 32 (non-coated portion 32A). It is good. The dimension α is not limited to this, but may be, for example, 113% or more, for example, 150% or less of the dimension in which the negative electrode active material layer 44 protrudes from the positive electrode active material layer 34. good. On the side of the insulating layer 36 that is not adjacent to the positive electrode active material layer 34, a non-coated portion 32A in which the positive electrode current collector 32 is exposed for current collection is provided.

The inorganic filler constituting such an insulating layer 36 does not soften or melt at a temperature of 600 ° C. or higher, typically 700 ° C. or higher, for example, 900 ° C. or higher, and can maintain insulation between positive and negative electrodes. A material having heat resistance and electrochemical stability can be used. Typically, it can be composed of the above-mentioned heat-resistant and insulating inorganic materials, glass materials, and composite materials thereof. Specific examples of such an inorganic filler include inorganic oxides such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ), and titania (TIO ₂ ), aluminum hydroxide, and silicon nitride. Examples thereof include metal hydroxides such as nitrides, calcium hydroxide, magnesium hydroxide and aluminum hydroxide, clay minerals such as mica, talc, boehmite, zeolite, apatite and kaolin, and glass materials. Among them, as the inorganic filler, boehmite (Al ₂ O ₃・ H ₂ O), alumina (Al ₂ O ₃ ), silica (SiO ₂ ), etc., which have stable quality and are inexpensive and easily available, are used. Is preferable, and boehmite having an appropriate hardness is more preferable. These may contain any one kind alone, or may contain two or more kinds in combination.

As the binder contained in the insulating layer 36, for example, various binders that can be used for the positive electrode active material layer can be preferably used. Among them, the binder is made of polyvinylidene from the viewpoint of appropriately forming the insulating layer 36 having an appropriate thickness while imparting the flexibility when collecting a plurality of positive electrode current collectors 32 to the insulating layer 36. A vinyl halide resin such as vinylidene (PVdF) can be preferably used. The proportion of the binder contained in the insulating layer 36 is typically 1% by mass or more, preferably 5% by mass or more, and may be 8% by mass or more, 10% by mass or more, and the like. The binder contained in the insulating layer 36 is typically, for example, 30% by mass or less, 25% by mass or less, 20% by mass or less, 18% by mass or less, and 15% by mass or less. .. As a typical example, it may be appropriately adjusted at 5 to 20% by mass. The insulating layer 36 may have a grain size of approximately 0.5 mg / cm ² or more, 0.7 mg / cm ² or more, 1 mg / cm ² or more, 1.5 mg / cm ² or less, and 1 It is preferable that the amount is 3 mg / cm ² or less, 1.2 mg / cm ² or less, and the like.

The insulating layer 36 is configured to prevent a short circuit between the positive electrode current collector 32 and the negative electrode active material layer 44 even when the secondary battery 1 is exposed to a high temperature environment of, for example, 150 ° C. .. 150 ° C. is a value significantly higher than the shutdown temperature designed for a separator having a general shutdown function. That is, the insulating layer 36 is parallel to the surface of the insulating layer evaluation sample formed in a square shape having a side of 5 cm and subjected to heat treatment at 150 ° C. for 1 hour. The heat shrinkage rate in the direction of is 13% or less. Although the insulating layer 36 is formed on the positive electrode current collector 32, its surface is not constrained by other members such as a separator. Therefore, the above-mentioned heat shrinkage rate is a value measured in an independent state of the insulating layer evaluation sample prepared in the same manner as the insulating layer 36. In other words, the heat shrinkage rate is measured in a state where the shrinkage in the plane direction of the insulating layer evaluation sample is not limited. For example, it is a heat shrinkage rate when a sample for evaluation of an insulating layer is placed on a top plate of a heating furnace, a sample case, or the like and heated in a state where the sample is released from a base material or the like. Therefore, the heat shrinkage rate of the sample for evaluation of the insulating layer, such as the state of being formed on the surface of the base material or the state of being sandwiched by a plate material or the like for simulating the laminated state inside the laminated electrode body. It is clearly distinguished from the heat shrinkage rate and the like measured under the condition that the shrinkage in the plane direction is restricted. The method for measuring the heat shrinkage of the insulating layer evaluation sample can be preferably measured by, for example, the method described in Examples described later.

When the heat shrinkage rate of the insulating layer evaluation sample is 13% or less, it can be said that even if the binder is softened or melted, further heat shrinkage is suppressed by the inorganic filler. Therefore, due to the presence of the inorganic filler, even if a physical force acts on the insulating layer 36, its deformation (shrinkage) is suppressed, and the insulating property can be maintained. The heat shrinkage rate is preferably 12% or less, more preferably 10% or less, and may be, for example, 8% or less, 6% or less, or 4% or less. The lower limit of the heat shrinkage rate may be 0%, but even if the heat shrinkage rate is 1% or more, for example, 2% or more, sufficient insulation can be maintained at high temperatures.

The shape of the inorganic filler is not particularly limited, and may be various shapes such as spherical, granular, plate-like, or fibrous. From the viewpoint of preferably achieving the above heat shrinkage rate, the inorganic filler is preferably composed of a powder containing plate-like particles. The plate-like particles have shape anisotropy. Since the particles constituting the inorganic filler are plate-shaped particles, as shown in FIG. 4A, the plate-shaped particles are inorganic in the insulating layer 36 so that the plane of the plate-shaped particles is substantially parallel to the surface of the insulating layer 36. The filler 36a can be plane-oriented. In such an insulating layer 36, in order for the inorganic filler 36a to move in the thickness direction, it is necessary to move all the inorganic fillers 36a existing in the moving direction, so that a large resistance acts. In the insulating layer 36, in order for the inorganic filler 36a to move in the plane direction, only the gap between the particles adjacent to each other in the plane direction can be easily moved, but in order to move further, the inorganic filler existing in the moving direction is present. Since it is necessary to move all 36a, a large resistance acts. As a result, even when the insulating layer 36 shrinks in the plane direction, a large resistance acts after the insulating layer 36 shrinks to some extent. As a result, when the inorganic filler 36a is composed of plate-like particles, the shrinkage rate (including the heat shrinkage rate) in the plane direction of the insulating layer 36 is relatively small and limited.

Here, the plate-like particles are significantly smaller in one dimension than the so-called spherical particles (for example, 80% or less, 50% or less, 40% or less, 30% or less), in other words. Flat and thin particles. Therefore, the plate-like particles may include particles expressed as, for example, scaly particles and the like. However, particles whose dimensions in the two dimensions are significantly smaller, such as rod-shaped and needle-shaped particles, are not included in the plate-shaped particles. When the inorganic filler contains plate-like particles, 50% or more (preferably 80% or more) of the particles constituting the inorganic filler have an aspect ratio of 1.2 or more (for example, 1.5 or more and 2 or more). It means that it is a plate-like particle of 2.5 or more and 3 or more). The ratio of the plate-like particles may be calculated based on the results of observing 100 or more inorganic filler particles in an electron microscope (typically, a transmission electron microscope (TEM); the same applies hereinafter) at an appropriate magnification. ..

The average aspect ratio of the inorganic filler is preferably 2.5 or more, and preferably 3 or more, for example, 4 or more, 5 or more, 6 or more, 7 or more, or 8 or more. The upper limit of the average aspect ratio of the inorganic filler is not particularly limited, and from the viewpoint of ease of handling and particle strength, an example of about 20 or less may be used as a guide. Further, the "average aspect ratio" in the present specification is an arithmetic mean of the ratio (diameter / thickness) of the diameter in a plan view to the thickness of the inorganic filler particles measured by observation with an electron microscope for 20 or more inorganic filler particles. The value. As the diameter, for example, the biaxial average diameter of the inorganic filler particles can be adopted. For the thickness, for example, an arithmetic mean value of the thickness of two or more points of the inorganic filler particles can be adopted.

On the other hand, the shape anisotropy of spherical particles is small. Therefore, when the particles constituting the inorganic filler are spherical particles, the spherical particles may randomly exist in the insulating layer 36 as shown in FIG. 4 (b). In the insulating layer 36, in order for the inorganic filler 36a to move in the plane direction, the inorganic filler 36a easily moves in the plane direction until it comes into contact with adjacent particles in the plane direction, and then when it comes into contact with the adjacent particles, the particles themselves. By moving the particles in the direction of the peripheral gaps, the particles can be further moved in the thickness direction or the surface direction. Further, when the particles come into contact with adjacent particles, the particles can be further moved in the plane direction by moving the contacted particles in the direction of the peripheral gaps. The resistance at this time may be smaller than the resistance when the plate-shaped particles move until the filled state of the spherical particles becomes high. As a result, when the inorganic filler 36a is composed of spherical particles, the shrinkage rate (including the heat shrinkage rate) in the surface direction of the insulating layer 36 becomes relatively large.

Although the thickness of the insulating layer 36 (average thickness; the same applies hereinafter) is not strictly limited, for example, when a metallic foreign substance is mixed between the positive electrode and the negative electrode, the positive electrode current collector 32 and the negative electrode active material due to the metallic foreign substance are used. It is preferable that the thickness is such that short-circuiting with the layer 44 can be sufficiently suppressed. From this point of view, the thickness of the insulating layer 36 may be 1 μm or more, preferably 3 μm or more, and more preferably 4 μm or more, for example. However, it is desirable that the insulating layer 36 has as little volume as possible because it may cause a decrease in workability of foil collection and welding. From this point of view, the insulating layer 36 may be 20 μm or less, for example, 18 μm or less, 15 μm or less, 10 μm or less (for example, less than 10 μm), or 8 μm or less, for example, 6 μm or less, 5 μm or less. For example, when the thickness of the insulating layer 36 is T1 and the thickness of the positive electrode active material layer is T2, the ratio of the thicknesses T1 to T2 (T1 / T2) is 1 or less, typically 1/2 or less. Yes, 2/5 or less is preferable, 1/3 or less is more preferable, 1/4 or less, 1/5 or less, and the like are more preferable. Further, from the viewpoint that the insulating layer 36 fully exerts its function, the ratio (T1 / T2) is preferably 1/10 or more, and may be, for example, 1/8 or more or 1/6 or more. The thickness T1 of the insulating layer 36 is the height of the insulating layer 36 from the surface of the positive electrode current collector 32, and does not include the thickness of the portion formed by superimposing the insulating layer 36 on the positive electrode active material layer 34. ..

The average particle size of the inorganic filler is not particularly limited. From the viewpoint of preferably forming the insulating layer 36 having the above thickness, the average particle size is typically 3 μm or less, preferably 2 μm or less, and for example, 1 μm or less. However, an inorganic filler that is too fine is not preferable because it is inferior in handleability and uniform dispersibility. Therefore, the average particle size of the inorganic filler is typically 0.05 μm or more, preferably 0.1 μm or more, for example 0.2 μm or more. This average particle size is a cumulative 50% particle size in the volume-based particle size distribution obtained by the laser diffraction / scattering method, as in the case of the positive electrode active material.

A preferred embodiment of the insulating layer 36 disclosed herein is characterized by the insulating layer 36 further comprising LPO. When the insulating layer 36 contains the LPO, the LPO is eluted from the insulating layer 36 at the time of overcharging, and a high-quality film that preferably contributes to the suppression of heat generation of the negative electrode 40 can be effectively formed on the surface of the negative electrode 40. According to the study by the present inventors, by providing an insulating layer 36 in the vicinity of the positive electrode current collector, which tends to be in a high potential state due to current concentration, and arranging the LPO on the insulating layer 36, the film derived from this LPO becomes the negative electrode. It was clarified that it was formed in a form capable of suppressing the exothermic reaction of the above. It should be noted that a configuration in which an insulating layer is provided at the positive electrode coating end is known (see, for example, Patent Document 1 and the like), but it has been so far that arranging the LPO on the insulating layer 36 is particularly effective for overcharge resistance. It is a new technical matter that is not known to. With such a configuration, further exothermic reaction on the surface of the negative electrode can be effectively suppressed, and for example, the overcharge resistance of the battery can be further enhanced.

When the insulating layer 36 contains LPO, the shape of the LPO and the like are not particularly limited. As the LPO, for example, an LPO having the same properties as that contained in the positive electrode active material layer 34 can be used. However, since LPO is relatively expensive, for example, the insulating layer 36 having the above thickness can be suitably formed, and when the battery reaches an overcharged state, it can be rapidly eluted into the electrolytic solution. preferable. From this point of view, the average particle size of LPO is typically 10 μm or less, preferably 8 μm or less, for example 5 μm or less, typically 1 μm or more, preferably 2 μm or more, for example 2.5 μm or more. May be.

When the average particle size of the inorganic filler is D1, the average particle size of the LPO in the insulating layer 36 is D2, and the average particle size of the positive electrode active material is D3, it is preferable that D1 and D2 <D3 are satisfied. It is preferable to satisfy <D2, and it is more preferable to satisfy D1 <D2 <D3. Since D1 and D2 <D3, for example, as shown in FIG. 4, when the insulating layer 36 is formed so as to overlap the end portion of the positive electrode active material layer 34, the insulation from the surface of the positive electrode current collector 32 is provided. It is possible to preferably suppress that the surface height position of the layer 36 is higher than the surface height position of the positive electrode active material layer 34 from the surface of the positive electrode current collector 32. Further, since D1 <D2, the strength of the insulating layer 36 is secured by the fine inorganic filler, and the insulating property is secured even when the LPO is eluted, so that the positive electrode current collector 32 and the negative electrode active material layer are secured. The short circuit with 44 can be suppressed more preferably. Further, the LPO can be easily arranged on the insulating layer 36 so as to elute.

The negative electrode 40 is configured by providing a negative electrode active material layer 44 on a negative electrode current collector 42. The negative electrode current collector 42 is provided with a non-coated portion 42A in which the negative electrode active material layer 44 is not formed for current collection and the negative electrode current collector 42 is exposed. The negative electrode active material layer 44 contains a negative electrode active material. Typically, the particulate negative electrode active materials may be bonded to each other by a binder (binder) and bonded to the negative electrode current collector 42. The negative electrode active material occludes lithium ions, which are charge carriers, from the electrolytic solution and releases them to the electrolytic solution during charging and discharging. As the negative electrode active material, various materials conventionally used as the negative electrode active material of the lithium ion secondary battery can be used without particular limitation. Preferable examples thereof include carbon materials typified by artificial graphite, natural graphite, amorphous carbon and composites thereof (for example, amorphous carbon-coated graphite), or materials that form an alloy with lithium such as silicon (Si). Lithium alloy (for example, Li _X M, M is C, Si, Sn, Sb, Al, Mg, Ti, Bi, Ge, Pb or P, etc., and X is a natural number), silicon compound (SiO, etc.), etc. Examples of the lithium-storing compound. The negative electrode 40 is, for example, a powdery negative electrode active material and a binder (for example, styrene butadiene copolymer (SBR), rubbers such as acrylic acid-modified SBR resin (SBR-based latex), carboxymethyl cellulose (CMC), etc. A negative electrode paste obtained by dispersing (for example, water or N-methyl-2-pyrrolidone, preferably water) in a suitable dispersion medium (for example, water or N-methyl-2-pyrrolidone) is supplied to the surface of the negative electrode current collector 42 and then dried. It can be produced by removing the dispersion medium. As the negative electrode current collector, a conductive member made of a metal having good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be preferably used.

The average particle size (D ₅₀ ) of the negative electrode active material particles is not particularly limited, and may be, for example, 0.5 μm or more, preferably 1 μm or more, and more preferably 5 μm or more. Further, it may be 30 μm or less, preferably 20 μm or less, and more preferably 15 μm or less. The ratio of the negative electrode active material to the entire negative electrode active material layer 44 is preferably about 50% by mass or more, preferably 90% by mass to 99% by mass, for example, 95% by mass to 99% by mass. When a binder is used, the ratio of the binder to the negative electrode active material layer 44 can be, for example, about 0.1 part by mass to 5 parts by mass with respect to 100 parts by mass of the negative electrode active material, and is usually about 0. .It is appropriate to use 5 parts by mass to 2 parts by mass. The thickness of the negative electrode active material layer 44 (which is the average thickness; the same applies hereinafter) can be, for example, 10 μm or more, typically 20 μm or more, and 80 μm or less, typically 50 μm or less. The density of the negative electrode active material layer 44 is not particularly limited, but is, for example, 0.8 g / cm ³ or more, typically 1.0 g / cm ³ or more, and 1.5 g / cm ³ or less, typically. Can be 1.4 g / cm ³ or less, for example 1.3 g / cm ³ or less.

The surface of the negative electrode active material layer 44 may be provided with a coating film (not shown) derived from LPO. This film may be formed by initial charging after battery assembly, or may be formed by overcharging. The film derived from LPO can be confirmed by detecting a phosphate ion ( ^{PO 43-} ₎ or phosphorus (P) component on the surface of the negative electrode active material layer. As an example, the negative electrode active material layer is punched out to a predetermined size, and the surface thereof is washed with an acidic solvent (for example, sulfuric acid) to ^{elute the phosphate ion (PO 43-} ₎ or phosphorus (P) component. Then, from this eluate, for example, by quantifying phosphorus atoms by inductively coupled plasma emission spectroscopy (ICP-OES: Inductively Coupled Plasma-Optical Emission Spectrometry) or by quantifying phosphate ions by ion chromatography, the negative electrode activity It is possible to grasp the existence of the LPO-derived film formed on the surface of the material layer and the amount of the film formed. The method for qualitative and quantitative analysis of phosphate ion ( ^{PO 43-3} ₎ or phosphorus (P) component is, for example, the above example or a known analytical chemistry method in consideration of the influence of additives in the electrolytic solution. Therefore, any person in the art can make an appropriate selection.

The separator 50 is a component that insulates the positive electrode 30 and the negative electrode 40 and provides a transfer path for the charge carrier between the positive electrode active material layer 34 and the negative electrode active material layer 44. Such a separator 50 is typically arranged between the positive electrode active material layer 34 and the negative electrode active material layer 44. The separator 50 may have a function of holding the non-aqueous electrolytic solution and a function of shutting down the movement path of the charge carrier at a predetermined temperature. Such a separator 50 can be suitably configured by a microporous resin sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Among them, the microporous sheet made of a polyolefin resin such as PE or PP preferably has a shutdown temperature in the range of 80 ° C to 140 ° C (typically 110 ° C to 140 ° C, for example 120 ° C to 135 ° C). It is preferable because it can be done. The shutdown temperature is a temperature at which the electrochemical reaction of the battery is stopped when the battery generates heat, and shutdown is typically expressed by melting or softening the separator 50 at this temperature. The separator 50 may have a single-layer structure composed of a single material, and two or more types of microporous resin sheets having different materials and properties (for example, average thickness, porosity, etc.) are laminated. (For example, a three-layer structure in which PP layers are laminated on both sides of the PE layer) may be used.

The thickness of the separator 50 (which is an average thickness; the same applies hereinafter) is not particularly limited, but can be usually 10 μm or more, typically 15 μm or more, for example, 17 μm or more. The upper limit can be 40 μm or less, typically 30 μm or less, for example, 25 μm or less. When the average thickness of the base material is within the above range, the permeability of the charge carrier can be kept good, and a minute short circuit (leakage current) is less likely to occur. Therefore, both input / output density and safety can be achieved at a high level.

The non-aqueous electrolyte solution typically dissolves or disperses a supporting salt as an electrolyte (for example, a lithium salt, a sodium salt, a magnesium salt, etc., and a lithium salt in a lithium ion secondary battery) in a non-aqueous solvent. It can be used without any particular limitation. Alternatively, it may be a so-called polymer electrolyte, a solid electrolyte, or the like, in which a polymer is added to a liquid non-aqueous electrolyte to form a gel. As the non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones used as an electrolytic solution in a general lithium ion secondary battery may be used without particular limitation. can. For example, specific examples thereof include chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), and cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). .. In particular, it is preferable to partially contain a solvent (for example, cyclic carbonate) that is decomposed in the acidic atmosphere of the positive electrode to generate hydrogen ions. Such a non-aqueous solvent may be fluorinated. Further, as the non-aqueous solvent, one kind may be used alone, or two or more kinds may be used as a mixed solvent. As the supporting salt, various types used in a general lithium ion secondary battery can be appropriately selected and adopted. For example, the use of lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, and LiCF ₃ SO ₃ is exemplified. Since the technique disclosed herein has the effect of suppressing heat generation during overcharging, for example, a lithium compound containing fluorine that is decomposed during overcharging to generate hydrogen fluoride (HF) is used as a supporting salt. When used, it is preferable because the effect of this technique is clearly exhibited. Such a supporting salt may be used alone or in combination of two or more. The supporting salt is preferably prepared so that the concentration in the non-aqueous electrolyte is in the range of 0.7 mol / L to 1.3 mol / L.

Further, the non-aqueous electrolyte may contain various additives and the like as long as the characteristics of the lithium ion secondary battery of the present invention are not impaired. As such an additive, it can be used as a gas generating agent, a film forming agent, or the like for one or more purposes of improving the input / output characteristics of the battery, improving the cycle characteristics, improving the initial charge / discharge efficiency, and the like. Specific examples of such additives include fluorophosphate (preferably difluorophosphate, for example, lithium difluorophosphate represented by _{LiPO 2} F2), lithium bis (oxalat) borate (LiBOB), and the like. Oxalato complex compounds can be mentioned. It is appropriate that the concentration of these additives with respect to the entire non-aqueous electrolyte is usually 0.1 mol / L or less (typically 0.005 mol / L to 0.1 mol / L).

The lithium ion secondary battery 1 shown in FIG. 1 uses a flat square battery case as the battery case 10. However, the battery case 10 may be a non-flat square battery case, a cylindrical battery case, a coin battery case, or the like. Alternatively, the lithium ion secondary battery 1 may be a laminated bag formed in a bag shape by laminating a metal battery case sheet (typically an aluminum sheet) and a resin sheet. Further, for example, the battery case may be made of aluminum, iron, an alloy of these metals, high-strength plastic, or the like. Further, in the lithium ion secondary battery 1 shown in FIG. 1, for example, a long positive electrode 30 and a negative electrode 40 are laminated in a state of being insulated from each other by two separators 50, and have a cross section centered on a winding shaft WL. The so-called winding type electrode body 20 in the form of being wound in an oval shape is provided. As shown in FIGS. 2 and 3, the width W1 of the positive electrode active material layer 34, the width W2 of the negative electrode active material layer 44, and the width W3 of the separator satisfy the relationship of W1 <W2 <W3. Further, the negative electrode active material layer 44 covers the positive electrode active material layer 34 at both ends in the width direction, and the separator 50 covers the negative electrode active material layer 44 at both ends in the width direction. Further, the insulating layer 36 covers the positive electrode current collector 32 in a region facing at least the end of the negative electrode active material layer 44 while being adjacent to the positive electrode active material layer 34. However, the electrode body 20 of the lithium ion secondary battery 1 disclosed herein is not limited to the wound type electrode body, and for example, a plurality of positive electrodes 30 and 40 are insulated and laminated by a separator 50, respectively. It may be a so-called flat plate laminated type electrode body 20 having a different form. Alternatively, it may be a single cell in which one positive electrode 30 and one negative electrode 40 are housed in a battery case.

The battery case 10 is typically composed of a case body 11 having an opening on one side and a lid member 12 for covering the opening. Similar to the battery case of a conventional lithium-ion battery, the lid member 12 is provided with a safety valve for discharging the gas generated inside the battery case to the outside, a liquid injection port for injecting an electrolytic solution, and the like. May be good. Further, the lid member 12 may typically be provided with a positive electrode terminal 38 for external connection and a negative electrode terminal 48 in a state of being insulated from the battery case 10. The positive electrode terminal 38 and the negative electrode terminal 48 are electrically connected to the positive electrode 30 and the negative electrode 40 via the positive electrode current collecting terminal 38a and the negative electrode current collecting terminal 48a, respectively, and are configured to be able to supply electric power to an external load.

Although the lithium ion secondary battery disclosed herein can be used for various purposes, it may have higher safety, for example, during repeated charging and discharging at a high rate, as compared with the conventional product. Further, these excellent battery performance and reliability (including safety such as thermal stability at the time of overcharging) can be compatible at a high level. Therefore, by taking advantage of these characteristics, it can be preferably used in applications that require high energy density and high input / output density, and applications that require high reliability. Examples of such applications include drive power supplies mounted on vehicles such as plug-in hybrid vehicles, hybrid vehicles, and electric vehicles. It should be noted that such a secondary battery can be typically used in the form of an assembled battery in which a plurality of such secondary batteries are connected in series and / or in parallel.

Hereinafter, as a specific example, the non-aqueous electrolytic solution secondary battery disclosed here was produced. It should be noted that the present invention is not intended to be limited to those shown in such specific examples.

[Shrinkage rate of sample for insulating layer evaluation]
A sample for evaluating the insulating layer was prepared for evaluating the insulating layer. First, as inorganic fillers, eight types of metal oxide powders having different shapes and compositions were prepared. That is, as shown in Table 1 below, the inorganic fillers of Examples 1 to 4 and 6 to 7 are plate-shaped boehmite powders having different aspect ratios of 8.5 to 1.2. The inorganic filler of Example 8 is a spherical boehmite powder having an aspect ratio of 1.1. The inorganic filler of Example 5 is a plate-shaped alumina powder having an aspect ratio of 3.2. The aspect ratio of each inorganic filler is an arithmetic average value of the aspect ratio calculated from the thickness of the particles and the biaxial average diameter measured by TEM observation for 20 or more filler particles for each sample.

As shown in Table 1 below, the inorganic filler (F) of each example and PVdF (B) as a binder are mixed at a ratio of F: B = 85 to 70:15 to 30 to serve as a dispersion medium. The paste for the insulating layer of Examples 1 to 8 was prepared by dispersing in N-methyl-2-pyrrolidone (NMP) and kneading. The solid content concentration of the insulating layer paste was approximately 20 to 24% by mass. The amount of binder was set to the amount required to keep the coating properties (for example, viscosity) of the paste due to the difference in the shape of the inorganic filler to a certain standard. This insulating layer paste was applied to the surface of a polyethylene terephthalate (PET) film with a mold release agent and dried to prepare an insulating layer for evaluation. The thickness of the insulating layer for evaluation in each example was about 5 μm, and the basis weight was about 1.2 to 0.7 g / cm ² .

The insulating layer of each prepared example was peeled off from the PET film to form a sheet-shaped insulating layer, which was punched into a square of 5 cm square to prepare a sample for evaluation. The evaluation sample was carefully stamped with a cross mark of 3 cm in length and width in the center, and a reference line was marked. This evaluation sample was heated by storing it in a constant temperature bath at 150 ° C. for 1 hour. Then, from the dimensions of the reference line measured before and after heating, the shrinkage ratio was calculated based on the following equation: shrinkage ratio (%) = (pre-heating dimension-post-heating dimension) ÷ pre-heating dimension × 100 ;. As the pre-heating dimension and the post-heating dimension of the reference line, the arithmetic mean of the dimensions measured for the two lines intersecting vertically and horizontally was adopted. The results are shown in Table 1 below.

[Short circuit test]
(Construction of non-aqueous electrolyte secondary battery for short circuit test)
In the same manner as above, evaluation samples (5 cm square) of the insulating layers of Examples 1 to 8 were prepared.
In addition, a non-aqueous electrolyte secondary battery was prepared by the following procedure. That is, a layered structure lithium nickel cobalt manganese-containing composite oxide (LiNi _1/3 Co _1/3 Mn _{1/3 O2} : NCM) as a positive electrode active material, acetylene black (AB) as a conductive auxiliary agent, and By blending polyvinylidene fluoride (PVdF) as a binder in a mass ratio of NCM: AB: PVdF = 90: 8: 2, and kneading with N-methyl-2-pyrrolidone (NMP) as a solvent. A positive paste was prepared. Then, the prepared positive electrode paste was applied to both sides of a long aluminum foil having a thickness of 12 μm as a positive electrode current collector and dried to obtain a positive electrode provided with a positive electrode active material layer. The positive electrode was provided with a non-coated portion in which a positive electrode active material layer was not formed along one end in the width direction for current collection.

Graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are mixed in a mass ratio of C: SBR: CMC = 98: 1: 1. Negative electrode paste was prepared by blending and kneading with ion-exchanged water. Then, the prepared negative electrode paste was applied to both sides of a long copper foil having a thickness of 10 μm as a negative electrode current collector and dried to obtain a negative electrode provided with a negative electrode active material layer. The negative electrode was provided with a non-coated portion in which a negative electrode active material layer was not formed along one end in the width direction for current collection.

A wound electrode body was constructed by stacking the positive electrode and the negative electrode prepared above so as to insulate each other via two separators to form a laminated body, and then winding the layers. At this time, as schematically shown in FIG. 5, the positive electrode active material layer at the positive electrode end on the winding start side is partially peeled off to expose the positive electrode current collector, and the insulating layer of any one of Examples 1 to 8 is evaluated. The sample (36S) and the L-shaped metal foreign matter piece (M) were arranged and wound to obtain a wound electrode body of Examples 1 to 8. The insulating layer evaluation sample and the metal foreign matter test piece were positioned approximately at the center in the width direction, height direction, and thickness direction of the wound electrode body. Further, the positive electrode and the negative electrode were overlapped so that the non-coated portion of the positive electrode and the non-coated portion of the negative electrode were located on opposite sides in the width direction. As shown in FIG. 6, the metal foreign matter pieces used are L-shaped with a side length of 1 mm, a width of 100 μm, and a height of 200 μm, so that the thickness directions of the metal foreign matter pieces coincide with the stacking direction. It was placed in the center of the insulating layer evaluation sample. As the separator, a porous sheet having a three-layer structure of PP / PE / PP was used.

As a battery case, a flat square battery case made of aluminum alloy was prepared. Then, the positive electrode non-coated portion and the negative electrode non-coated portion of the wound type electrode body of each example are connected to the positive electrode terminal and the negative electrode terminal of the battery case, respectively, and the case is housed together with the non-aqueous electrolyte solution and then sealed. As a result, a non-aqueous electrolyte secondary battery for evaluation of Examples 1 to 8 was obtained. As the non-aqueous electrolytic solution, a mixed solvent containing ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of EC: EMC: DMC = 3: 3: 4 as a supporting salt. LiPF ₆ was dissolved at a concentration of 1 mol / L.

(Measurement of voltage drop at short circuit and maximum temperature reached)
The secondary batteries of each example are charged with a constant current (CC) at a rate of 1 / 3C until the voltage reaches 4.1V under a temperature environment of 25 ° C., and then a constant voltage until the current reaches 1 / 50C. (CV) Charged. As a result, the secondary batteries of each example were activated. Then, a constant current (CC) was discharged at a rate of 1 / 3C until the voltage became 3V. Next, the activated secondary battery was charged with a constant current (CC) at a rate of 1/3 C until the state of charge (SOC) reached 90% in a temperature environment of 25 ° C. Here, "1C" means a current value that can charge the battery capacity (Ah) predicted from the theoretical capacity of the active material in one hour.

A thermocouple was attached to the central part of the outside of the battery case of the secondary battery adjusted to SOC 90%. While measuring the voltage between the external terminals of the secondary battery, the secondary battery was forcibly short-circuited by pressing the portion where the metal foreign matter piece was arranged from the outside of the battery case under predetermined conditions. For the pressing, it was determined that a single-layer short circuit (small short circuit) occurred due to a decrease in the voltage between the terminals by 2 mV, and the pressing was stopped when the single layer short circuit (decreased by 2 mV) was confirmed. Then, the voltage drop due to the slight short circuit was calculated by measuring the voltage 100 seconds after the short circuit. In addition, the temperature of the secondary battery after the short circuit was observed, and the maximum temperature reached was investigated. These results are listed in the relevant column of Table 1.

(Evaluation: Heat shrinkage rate)
As shown in Table 1, it was found that the heat shrinkage rate of the insulating layer when heated to 150 ° C. differs depending on the composition and form of the inorganic filler constituting the insulating layer. Specifically, as shown in Examples 1 to 7, when the aspect ratio of the plate-shaped inorganic filler constituting the insulating layer was changed from 1.2 to 8.5, the smaller the aspect ratio, the more the insulating layer. It was confirmed that the heat shrinkage rate increased. In the insulating layer formed by the slurry coating method, the plate-shaped inorganic filler is oriented so that its surface direction is substantially parallel to the surface direction of the insulating layer (in other words, the base material) and overlaps in the thickness direction. It is placed in the state of being. The larger the aspect ratio of the inorganic filler, the smaller the number of adjacent particles, in other words, the gaps between the particles, in the plane of the insulating layer per unit area. Therefore, even if a force in the plane direction acts on the plate-shaped particles in the insulating layer, it is difficult for the plate-shaped particles to move more than the particle gap after colliding with the adjacent particles in the plane. As a result, even when the binder in the insulating layer is softened or melted by being exposed to a high temperature, the movement of the inorganic filler in the insulating layer in the plane direction is suppressed. Therefore, it is considered that the heat shrinkage rate becomes smaller as the insulating layer contains the inorganic filler having a larger aspect ratio. The shrinkage rate of such an insulating layer is reduced to 13% or less even when the secondary battery generates heat up to about 150 ° C., for example. Therefore, by arranging this insulating layer in a region along the end of the positive electrode active material layer and facing the negative electrode active material layer, even if the separator is exposed to a high temperature such as thermal shrinkage, this It is possible to suitably suppress a short circuit between the insulating layer and the negative electrode facing the negative electrode. From the comparison of Examples 4 and 5, it was found that the heat shrinkage rate can be suppressed to a low level by using alumina having a higher melting point and excellent fire resistance than using boehmite as the inorganic filler.

On the other hand, the granular inorganic filler has almost no shape anisotropy. Further, spherical particles having a small aspect ratio have a large number of adjacent particles per unit area in the plane of the insulating layer, in other words, a large number of gaps between the particles. Therefore, when a force acts on the spherical particles in the insulating layer in the plane direction, the spherical particles collide with the adjacent particles in the plane and then move more easily together with the collided particles. Also, the spherical particles can move so as to overlap (that is, in the thickness direction) along the surface of the collided particles, and can move more than the particle gaps. Plate-shaped inorganic filler particles having a small aspect ratio can also behave similarly to these spherical particles. As a result, when the binder in the insulating layer is softened or melted, it is considered that the inorganic filler in the insulating layer easily moves in the plane direction and the insulating layer is easily heat-shrinked.

From the comparison between Examples 7 and 8, it was confirmed that when the amount of the binder in the insulating layer is large, the binder softens during heating and the inorganic filler easily moves, resulting in a high heat shrinkage rate. From this, it can be said that the ratio of the binder in the insulating layer is preferably less than 30% by mass, preferably 25% by mass or less, 20% by mass or less, for example, 15% by mass or less.

(Evaluation: Short circuit test)
In addition, from the results of the short-circuit test, it was confirmed that the voltage drop of the battery at the time of forced short-circuit and the maximum temperature reached differ due to the difference in the configuration of the insulating layers arranged side by side in the positive electrode active material layer. .. The amount of voltage drop and the maximum temperature reached are well correlated. It was also found that the voltage drop and the maximum temperature reached correlate well with the heat shrinkage of the insulating layer, and that the smaller the heat shrinkage of the insulating layer, the lower the voltage drop and the maximum temperature reached. In other words, it was found that the smaller the heat shrinkage rate of the insulating layer, the more the amount of short circuits caused by the metal foreign matter pieces can be reduced, and the short circuits can be suppressed to a mild level.
The width of the portion where the metal foreign matter piece abuts on the insulating layer, which is the cause of the short circuit, is as small as 100 μm. Therefore, even if the battery is in a state where the binder is not softened or melted at room temperature, when the metal foreign matter pieces are physically pressed against the insulating layer in the thickness direction, the inorganic filler particles having a small aspect ratio move in the plane. Easy to do and easy to push away. As a result, the metal foreign matter pieces bite into the insulating layer and easily reach (short-circuit) the current collector. Especially in the insulating layer of Example 7 in which the amount of binder is large, the inorganic filler particles move more easily in the plane, the metal foreign matter pieces easily reach the current collector, a short circuit occurs in a wide area, and the voltage drop amount is large. It is thought that it became. As a result, it is considered that the amount of heat generated by the short circuit increases and the short circuit expands to a level where safety is reduced.

On the other hand, it is considered that the inorganic filler particles having a large aspect ratio are difficult to move in the plane of the insulating layer, and it is difficult to form a path for contacting the current collector even if a metal foreign matter piece is pressed against the particles. As a result, it is considered that the metal foreign matter pieces are difficult to reach the current collector easily, the area is small even if a short circuit occurs, and the short circuit can be reduced to a mild level. As a result, it is possible to suppress an increase in the short-circuit area due to heat generation due to a short circuit and further heat generation of the battery due to this. From this, it was found that the insulating layer having a small heat shrinkage rate can suitably suppress or reduce short circuit due to metal foreign matter or the like even at room temperature.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above.

1 Secondary battery 30 Positive electrode 32 Positive electrode current collector 34 Positive electrode active material layer 36 Insulation layer 40 Negative electrode 42 Negative electrode current collector 44 Negative electrode active material layer 50 Separator

Claims (6)

A positive electrode, a negative electrode, a separator that insulates the positive electrode and the negative electrode, and a non-aqueous electrolyte are provided.
The positive electrode is
Positive current collector and
A positive electrode active material layer provided on a part of the surface of the positive electrode current collector and containing a positive electrode active material,
An insulating layer that is another part of the surface of the positive electrode current collector and is provided adjacent to the positive electrode active material layer.
Equipped with
The insulating layer contains an inorganic filler and a binder, and
For an insulating layer evaluation sample formed in a square shape with a side of 5 cm, the heat shrinkage rate in one direction parallel to the surface of the insulating layer evaluation sample when heat-treated at 150 ° C. for 1 hour is Non-aqueous electrolyte secondary battery with 13% or less. The non-aqueous electrolyte secondary battery according to claim 1, wherein the inorganic filler contains plate-like particles. The non-aqueous electrolyte secondary battery according to claim 2, wherein the inorganic filler contains plate-like particles having an average aspect ratio of 3 or more. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the inorganic filler is at least one of boehmite powder and alumina powder. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the ratio of the binder to the total of the inorganic filler and the binder is less than 30% by mass in the inorganic filler. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the insulating layer has an average thickness of 10 μm or less.

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